Carbonless copy paper usually consists of two or more sheets of coated paper. For example, when three sheets of paper are used, the top sheet has a coated back (CB) which is formed by coating the back or underside of the top sheet with a hydrophilic colloid solution in which are dispersed microcapsules containing a colorless electron donating chromogenic color former. The middle sheet is coated on its front and back (CFB). The coating on the front or upperside contains an electron accepting color developer, and the coating on the back or underside is the same as the microcapsule-containing coating on the top sheet. The bottom sheet is coated on its upperside or front (CF) with the same composition that is on the front of the middle sheet. Detailed disclosures of carbonless copy papers are included in U.S. Pat. Nos. 3,554,781; 4,154,462; 4,337,968; 4,352,855; 4,371,634; and 4,411,451.
In order to form an image with carbonless copy paper, the system utilizes the color forming reaction that takes place between the electron donating chromogenic material or color former in the coated back and the electron accepting acidic reactant material or color developer in the coated front (CF). The reactants are isolated from each other by microencapsulating one of them until an image is desired. When pressure is applied to the carbonless copy paper, the microcapsules are ruptured and a reaction occurs between the color former and color developer to provide the desired image. In the preferred type of carbonless copy paper, the color former is the reactant that is encapsulated by being dissolved in oil and microencapsulated prior to being used in the coating on the coated back.
A preferred method of making microcapsules containing color former reactants is the complex coacervation procedure described in U.S. Pat. No. 2,800,457. In this procedure an oil containing a color former is dispersed in two colloid materials that have opposite charges, and at least one is gellable. Coacervation of the hydrophilic colloid solution around each oil droplet is caused by dilution and adjusting the pH to the acidic range. The coacervate around each oil droplet is gelled by cooling and hardened by addition of a suitable hardening agent, and the pH of the mixture is adjusted to the alkaline range.
Carbonless copy papers that are produced from multi-nuclear and clustered microcapsules are subject to premature rupturing of the microcapsules during handling or during post-coating conversion processes, especially in the printing of business forms. Also, such copy papers have very poor humidity and heat resistance, and those copy papers have only very limited utility in hot, humid climates. Much effort has been expended to solve these problems by preparing coacervation microcapsules that are mononuclear and cluster-free. For example, maleic anhydride copolymers or carboxymethyl cellulose have been incorporated into the walls of the microcapsules, but these prior art procedures have not been successful, although reaction conditions were strictly maintained during the coacervation procedure.
Also, there are unsolved problems with the coated fronts that are used in prior art carbonless copy papers. Usually an acidic clay or an acidic polymer is used in the coated front. Images on acidic clay-coated paper are not sufficiently permanent and they have only limited resistance to moisture. Images on acidic polymer-coated paper are time and temperature dependent and frequently too slow. Better results are sometimes obtained by using a combination of acidic clay and acidic polymer, but image formation is still too slow, especially at low temperatures such as at 5.degree. C.